1. Field of the Invention
This invention relates to thermal ink jet printing devices and, more particularly, to improved bubble generating heating elements or transducers for thermal ink jet devices, which results in longer heating element lifetimes and a more cost effective manufacturing process.
2. Description of the Prior Art
Thermal ink jet printing is generally a drop-on-demand type of ink jet printing which uses thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. A thermal energy generator or heating element, usually a resistor, is located in the channels near the nozzle a predetermined distance therefrom. The resistors are individually addressed with an electrical pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
The environment of the heating element during the droplet ejection operation consists of high temperatures, thermal stress, a large electrical field, and a significant cavitational stress. Thus, the need for a cavitational stress protecting layer over the heating elements was recognized early, and one very good material for this purpose is tantalum (Ta), as is well known in the industry.
The previous art of using an electrically insulating layer between the heating elements and the Ta protecting layer, such as, for example, silicon dioxide or plasma deposited silicon nitride presents several problems. These problems include (1) the poor thermal conductivity of silicon dioxide or silicon nitride, so that the heating element must be much hotter than the Ta layer, making the heating element thermally inefficient, and (2) the fact that the delineation of Ta requires plasma etching in CF.sub.4 /O.sub.2 mixtures. Unlike plasma etching of many other materials, the volatile gases evolved have low vapor pressure and are difficult to pump off. The Ta process is also very sensitive to moisture, both in the plasma etch chamber and in the Ta deposition chamber. The etch process sometimes leaves a residue which can result in poor aluminum (Al) addressing electrode adhesion to the heating elements as well as leading to wire bond failure at the Al electrode terminal interface.
The present inventive heating element structure and method of fabrication eliminates all of these problems, as discussed later. A review of the known prior art, such as identified below, recognized some problems with silicon dioxide and plasma deposited silicon nitride, such as life shortening pinholes and the like, but could not find adequate solutions.
U.S. Pat. No. 4,259,564 to Ohkubo et al discloses use of thin film manufacturing process to achieve improved resolution in integrated thermal printheads while retaining some less costly thick-film manufacturing processes to keep the over all printhead cost down. The printhead comprises a dielectric substrate, thick-film lead electrode layers with insulating means therebetween, thin-film resistive layers, thin-film lead electrode layers, and two protective overlayers; viz., an oxidation preventive film of SiO.sub.2, Al.sub.2 O.sub.3, or the like and a protective wear resistance layer made of Ta.sub.2 O.sub.5, Al.sub.2 O.sub.3, or the like covering the oxidation preventive film.
U.S. Pat. No. 4,450,457 to Miyachi et al discloses the use of a double passivation layer for the electrodes to prevent galvanic corrosion of electrodes caused by both pinholes in silicon dioxide sputtered layer used generally for passivation and by dielectric breakdown of the passivation between the Al electrodes and the tantalum cavitational protecting overlayer. The double passivation layer includes a layer of material such as an inorganic oxide or nitride (e.g., silicon dioxide or silicon nitride) and a second layer formed by plasma polymerization of various organic monomers as well as other methods of fabrication.
U.S. Pat. No. 4,577,202 to Hara discloses a double passivation layer for the electrodes wherein an organic passivation layer is formed over the inorganic passivation layer instead of the reverse order of passivation in U.S. Pat. No. 4,450,457.
U.S. Pat. No. 4,567,493 to Ikeda et al discloses a thermal ink jet printer similar to that of U.S. Pat. No. 4,577,202 except that the cavitational protective layer is only over the heating element.
U.S. Pat. No. 4,513,298 to Scheu discloses a thermal ink jet printhead employing a resistive element comprised of phosphorus-diffused silicon or polycrystalline silicon and a passivation structure including a layer of silicon nitride. The silicon nitride is formed by plasma enhanced chemical vapor deposition. An upper layer, the one in contact with the ink and on which the ink bubble collapses, is silicon carbide. This upper layer is formed on the lower silicon nitride passivating layer. Unfortunately, this passivation technique is not as free from defects such as pinholes and compromise protection of resistive elements and electrodes.
U.S. Pat. No. 4,535,343 to Wright et al reduces the pinhole problems of the U.S. Pat. No. 4,513,298 patent by using a resistive structure formed of tantalum nitride with electrical conductors of aluminum. The resistor structure between the electrical conductors is exposed and subjected to a reactive oxygen atmosphere. This results in the oxidation of the exposed surface portions of the aluminum conductors to anodize and form a surface film of Al.sub.2 O.sub.3 thereon. At the same time the oxygen reacts with the exposed resistor structure to form a smooth defect-free passivation layer of tantalum pentoxide or tantalum oxynitride. Also disclosed is a thermal insulating barrier of silicon dioxide.
U.S. Pat. No. 4,532,530 to Hawkins discloses a thermal ink jet printhead having heating elements produced from doped polycrystalline silicon. Glass mesas thermally isolate the active portion of the heating element from the silicon supporting substrate and from electrode connecting points.